The present invention relates to apparatus and methods for controlling the defrosting of the outdoor heat exchanger of heat pumps or similar closed temperature conditioning systems which bridge indoor and outdoor environments, wherein a compressor is employed to circulate a refrigerant fluid through the system.
It is well known that systems such as heat pumps or the like operate on a reverse cycle principle. In general, such a system includes indoor and outdoor heat exchange coils which are exposed to their respective ambient conditions. When the temperature of indoor air is to be raised (heating mode), the compressor pumps refrigerant fluid through the indoor heat exchange coil which becomes warm and, subsequently, by way of expansion means, through the outdoor heat exchange coil, which heat-exchanges with outdoor air. The outdoor coil functions as a refrigerant evaporator by absorbing heat from the outdoor air and thus becomes even colder than the outdoor air.
Since the outdoor heat exchange coil under these conditions operates at a temperature less than ambient outdoor air, ambient moisture condenses on the outdoor heat exchanger and, depending upon conditions, can freeze to cause a layer of frost or ice to form on the outdoor heat exchanger. This layer of frost or ice acts as an insulator between the outdoor ambient air and the outdoor heat exchanger, and impairs effective heat transfer to the outdoor heat exchanger. Under such conditions, system efficiency is greatly reduced.
Insofar as the need to defrost is concerned, it makes no difference whether the frozen condensate is termed "frost" or "ice". Accordingly, the two terms are employed interchangeably herein.
A variety of techniques are known, appropriate to various specific types of systems, for effecting a defrosting operation once excessive frosting is detected. Perhaps the simplest technique is to merely interrupt compressor operation, whereupon some heat from the indoor side inevitably migrates to the outdoor side, and may very well be sufficient to melt the frost. In other cases, heat is actively supplied, such as by employing electrical resistance heaters, operating the outdoor fan in the event outdoor ambiant temperature is above 32.degree. F., and operating the system in the opposite (indoor cooling) mode whereby hot compressed refrigerant is pumped into the outdoor heat exchanger which, at that point, is functioning as a condensor.
While a variety of straightforward techniques are known for supplying heat to defrost the outdoor heat exchanger when necessary, what is not so straightforward is determining when to initiate a defrosting operation. In a relatively simple conventional system, a temperature sensor is placed at the base of the outdoor heat exchanger, and is monitored during operation. When the sensed temperature falls below a predetermined threshhold, for example 10.degree. F., a defrosting operation is initiated. In a typical system thus controlled, periodic defrosting occurs at outdoor ambient temperatures from 45.degree. F. and below.
While such a simple system is relatively straightforward and serves the desired function, it is not optimum for all conditions, Clearly, overall efficiency of operation requires that defrosting occur when necessary, but only when necessary. One difficulty with a simple system is that ambient temperature has a significant effect on the sensitivity of the sensor to frost. Moreover, ambient conditions, which vary widely, affect the amount and rate at which frost builds on the outdoor heat exchanger. For example, the humidity of ambient air determines in some measure the rate of frost build up. Further, precipitation such as snow or freezing rain will affect the amount of frost deposited on the outdoor coil.
An effective frost control system must be capable of operating under a variety of ambient outdoor conditions in order to defrost the outdoor coil as necessary so as to operate the heat pump system at high efficiency.
Accordingly, various more elaborate systems, typically involving a plurality of sensors, have been proposed. One such system is described in Noland et al U.S. Pat. No. 4,102,391. The Noland et al system employs a pair of thermostats having capillary tube sensing elements, each of which extend so as to be influenced by temperatures in two different parts of the system.
Another example, particularly relevant in the context of the present invention, is disclosed in Pohl U.S. Pat. No. 4,215,554. In the system of that patent, a plurality of temperature sensors are employed, a particular pair of which comprise one sensing element positioned proximate the outdoor heat exchanger coil, preferably in direct contact with the bare coil, or between the coil fins, and another temperature sensing element exposed to the ambient outdoor air conditions prevailing near the outdoor heat exchanger, but spaced sufficiently far from it so as not to be affected by frost build-up thereon. A defrost cycle for the outdoor heat exchanger is initiated as a function of the temperature differential between the temperature of the outdoor heat exchanger and that of the ambient outdoor air, the precise temperature differential at which defrosting is triggered varying as a function of outdoor ambient air temperature.
Another decision to be made is when to terminate a defrosting operation. In a typical simple system employing a sensor located at the base of the outdoor heat exchanger, a decision is made to stop defrosting when the sensor temperature reaches, for example, 36.degree. F., which represents a significant rise in temperature above the 32.degree. F. latent heat of ice point.
The multi-sensor systems are typically more effective in adapting themselves to different ambient conditions. However, such systems are relatively costly insofar as the temperature sensors are relatively costly, since they need to be well-insulated, moisture-proof, and reliable over extended periods of time under occasional severe ambient conditions. Moreover, in a microprocessor-based system, analog-to-digital conversion of readings from a plurality of separate temperature sensors leads to added system cost and complexity.